The present invention relates to a color copier, color printer, color facsimile machine or similar electrophotographic color image forming apparatus.
Predominant color image forming apparatuses available today are generally classified into two types, i.e., one which transfers monocolor images formed on a photoconductive drum sequentially to a sheet one over the other, and the other which forms a composite color image on the drum and then transfers it to a sheet collectively. The problem with the sequential transfer type apparatus is that when use is made of a postcard or similar sheet of small size or a thick sheet, it is difficult to wrap the sheet around a transfer drum. Therefore, the material and size of sheets applicable to this type of apparatus are limited. Another problem with such an apparatus is that the image forming area available for the sheet is narrow since the sheet wrapped around a transfer drum is usually clamped at the edge thereof. The other type of apparatus, i.e., collective transfer type apparatus is disclosed in Japanese Patent Laid-Open Publication No. 106557/1992 by way of example. This type of apparatus has a problem that the steps of charging the surface of the drum over a toner image formed thereon, exposing the drum and developing the resulting latent image have to be repeated a plurality of times, making it difficult to maintain chargeability constant. In addition, this type of apparatus is lower in image quality than the sequential transfer type apparatus using the transfer drum.